1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery employing a nonaqueous electrolyte.
2. Description of the Prior Art
Nonaqueous electrolyte secondary batteries with nonaqueous electrolytic solutions composed of organic solvents and electrolytes are characterized by less self-discharge currents and high operating voltages, and available for long-term use. Heretofore, nonaqueous electrolyte secondary batteries are widely used as power supplies for use in electronic wrist watches and also as memory backup power supplies because they are highly reliable and can be used over a long period of time.
There has been a growing demand for portable power supplies for use in video cameras, compact audio devices, microcomputers, etc. To meet such a demand, attention has been drawn to rechargeable nonaqueous electrolyte secondary batteries for use as lightweight, large capacity power supplies that can be used economically over a long period of time.
Nonaqueous electrolyte secondary batteries that have been proposed have an cathode of lithium or lithium alloy and an anode made of an active material of MnO.sub.2, TiO.sub.3, MoS.sub.2, V.sub.2 O.sub.5, WO.sub.3, LiCoO.sub.2, or the like.
The nonaqueous electrolyte secondary battery which has a cathode made of a carbon material that can be doped or undoped with lithium and an anode made of a compound oxide of lithium and cobalt is particularly promising because its voltage is high, it can a high energy density, and its cyclic performance is much better than nonaqueous secondary batteries whose anode is made of metallic lithium or lithium alloy.
Nonaqueous electrolyte secondary batteries employ electrolytes of LiAsF.sub.6, LiPF.sub.6, LiBF.sub.4, LiC1O.sub.4, etc. It has been reported that use of LiPF.sub.6 as the electrolyte gives the electrolytic solution particularly excellent charging/discharging characteristics.
In the case where a nonaqueous electrolyte secondary battery employs an electrolyte of LiPF.sub.6 or LiBF.sub.4, however, the electrolyte is decomposed, dissociating a hydrofluoric acid which is a free acid, with the result that the discharge capacity of the battery is lowered.
More specifically, if metallic lithium is used as a cathode active material, then since the free acid reacts with metallic lithium thereby to produce lithium fluoride, the discharge capacity is not greatly affected. If LiPF.sub.6 or the like is used as an electrolyte, however, the active material is dissolved by the hydrofluoric acid, and the charging/discharging characteristics of the battery are reduced in repeated charging/discharging cycles. When the battery is used at high temperature, this tendency is stronger because the generation of the free acid is accelerated.
It has been proposed to add various additives to electrolytic solutions for solving the above problems to increase the stability of the electrolytic solutions (see, for example, Japanese Laid-Open Patent Publication No. 61-208758). However, the addition of various additives to electrolytic solutions has proven unsatisfactory as they largely affect the battery performances.